A gear transmission suitable for driving a joint of a multi joint robot is taught in Japanese Patent Application Publication H01-169154 (Patent Document 1). The gear transmission of Patent Document 1 comprises an internal gear member and a carrier. An internal gear is formed on an inner circumference of the internal gear member. The carrier is positioned coaxially with an axis of the internal gear and is rotatably supported on the internal gear member. An external gear meshing with the internal gear is supported on the carrier in an eccentrically rotatable manner. The internal gear and the external gear have a different number of teeth. The external gear rotates eccentrically around the axis of the internal gear while remaining meshed with the internal gear. At this time, the external gear rotates relative to the internal gear in accordance with the difference in the number of teeth of the external gear and the internal gear. When the external gear rotates relative to the internal gear, the carrier supporting the external gear rotates relative to the internal gear. This gear transmission achieves a gear reduction ratio in accordance with the difference in the number of teeth of the external gear and the internal gear.
The gear transmission of Patent Document 1 drives the joint of the multi joint robot. Consequently, the gear transmission of Patent Document 1 rotates a member, which is positioned closer to a robot tip side than the joint, relative to a member positioned closer to a robot base-side than the joint. In the multi joint robot, a plurality of joints is arranged from its base to its tip. A motor is required to drive a joint. If the motor is positioned in the vicinity of each joint, the tip of the robot becomes heavy. Accordingly, a configuration may be employed in which the motors are positioned as close to the robot base as possible, and each of the joints is connected with the respective motors by a torque transmission member such as a shaft, etc.
In the gear transmission of Patent Document 1, a through-hole extending through the carrier is formed along the axis of the internal gear. The shaft is passed through this through-hole, and torque can be transmitted from the motor, which is positioned closer to the robot base-side than the gear transmission, to the joint which is positioned closer to the robot tip side than the gear transmission. Therefore, the motor, which drives the joint positioned closer to the robot tip side than the gear transmission, can be positioned closer to the robot base-side than the gear transmission. The weight of the robot tip can thereby be reduced.